It depends on the plec. For plecs other than Panaque spp., use whatever wood you want. Even though they ingest it, they don't digest it, and all it does is act as a kind of fibre.
Panaque spp. on the other hand digest the wood they eat. I'm not altogether convinced it matters what kind of wood, since all wood is made of the same chemical, lignin, and all Panaque spp. use bacteria in their guts to break this stuff down. I know that some people have strong opinions on this, and maybe even the catfish have preferred flavours of wood! My Panaque spp. has regular bogwood to eat (the generic dark brown stuff) as well as a bunch of waterlogged sticks and branches I pulled out of a local stream. She loves the variety, I think, and tries a little of each through the day.
Cheers, Neale